The present invention relates to a device for producing drilled holes with an undercut.
More particularly, it relates to such a device which is used for producing drilled holes with an undercut in facing panels of ceramics, stone, concrete or similar materials and has a drill shank mounted in a swivel bearing and swivelling means for displacing the drill shank into a gyratory movement about a central line of drilling.
Devices of the above mentioned general type are known in the art. In order to produce drilled holes with an undercut, such a device has a bearing bush provided with a concave depression in which a drilling tool with a collar is supported and mounted so as to swivel. During swivelling out of the drilling tool and simultaneously performing a stirring movement, the lateral cutting edges of the drilling tool ream out and undercut in the region of the bottom of the drilled hole. An expansible plug with an expansible sleeve can be inserted into the thusly formed undercut with an interlocking fit.
The above described drilled holes with an undercut can also be made in facing panels or similar structures. But in this case the undercuts have to be made very exactly so that when an expansible anchor is inserted and expanded, the expansion pressure is not too great. The reason is that too great an expansion pressure can result in a part of the facing panel breaking away. In order to produce an exact undercut, the above mentioned tools are designed so that an automatic machine-controlled swivelling-out action can be performed. Furthermore, drilling tools with a drilling head set with diamond chips are used. When the drilling tool clamped in alignment with the drive axis of the driving machine is inserted into the facing panel, the entire end face of the drilling head having the shape of a truncated cone comes into engagement with the panel. This requires a high application pressure of the drilling tool, which leads to premature blunting of the diamond chips due to the low cutting speed at the center of the end face of the drilling tool. As a result, the service life of the drilling tool is considerably reduced.
The German document DE-A1 41 19 350.4 discloses a device for producing drilled holes with an undercut in facing panels in which the drilling machine and therefore the drill bit attached to it for producing the undercut are displaced away from the center line and positively guided along a cam plate during the reaming-out operation. Swivelling is provided by hand and for this reason the gyratory movement of the drill bit during the reaming out operation has only a low speed.